From: Eric Leblond <eric@xxxxxxxxx> Date: Fri, 7 Dec 2012 19:56:01 +0100 Wireless folks, please take a look. The issue is that, under the circumstances listed below, we get SKBs in the AF_PACKET input path that are shared. Given the logic present in ieee80211_deliver_skb() I think the mac80211 code doesn't expect this either. More commentary from me below: > This patch is adding a check on skb before trying to defrag the > packet for the hash computation in fanout mode. The goal of this > patch is to avoid an kernel crash in pskb_expand_head. > It appears that under some specific condition there is a shared > skb reaching the defrag code and this lead to a crash due to the > following code: > > if (skb_shared(skb)) > BUG(); > > I've observed this crash under the following condition: > 1. a program is listening to an wifi interface (let say wlan0) > 2. it is using fanout capture in flow load balancing mode > 3. defrag option is on on the fanout socket > 4. the interface disconnect (radio down for example) > 5. the interface reconnect (radio switched up) > 6. once reconnected a single packet is seen with skb->users=2 > 7. the kernel crash in pskb_expand_head at skbuff.c:1035 > > [BBB55:744364] [<ffffffff812a2761>] ? __pskb_pull_tail+0x43x0x26f > [BB8S5.744395] [<ffffffff812d29Tb>] ? ip_check_defrag+ox3a/0x14a > [BBB55.744422] [<ffffffffB1344459>] ? packet_rcv_fanout+ox5e/oxf9 > [BBBS5.7444S0] [<ffffffffB12aaS9b>] ? __netif_receive_skb+ox444/ox4f9 > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? netif_receive_skb+ox6d/0x?3 > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? ieee80211_deliver_skb+0xbd/0xfa [mac80211] > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? ieee80211_rx_h_data+0x1e0/0x21a [mac80211] > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? ieee80211_rx_handlers+0x3d5/0x480 [mac80211] > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? __wake_up > [BBB55.T4447B] [<ffffffffB12aa?e1>] ? evdev_eventr+0xc0/0xcf [evdev] > > Signed-off-by: Eric Leblond <eric@xxxxxxxxx> So if we look at ieee80211_deliver_skb(), it has code to deal with unaligned packet headers, wherein it memoves() the data into a better aligned location. But if these SKBs really are skb_shared(), this packet data modification is illegal. I suspect that the assumptions built into this unaligned data handling code, and AF_PACKET, are correct. Meaning that we should never see skb_shared() packets here. We just have a missing skb_copy() somewhere in mac80211, Johannes can you please take a look? Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
